In recent years a reduction in processing time has been desired earnestly in the photographic industry, so the development of silver halide photographic materials suitable for rapid processing has been a pressing need.
Silver halide emulsions having a high silver chloride content (which are simply abbreviated as high chloride content emulsions, hereinafter) are responsive to this need because they progress most rapidly in development of the light-sensitive silver halides.
Compared with silver halide emulsions having a high bromide content, however, high chloride content emulsions have a serious defect in that it is difficult for them to have high sensitivity because they tend to generate fog when subjected to chemical sensitization using a sulfur sensitizer, a selenium sensitizer, a gold or noble metal sensitizer, a reduction sensitizer or a mixture of two or more thereof, which is usually carried out in order to attain the desired sensitivity, the desired gradation and so on. In addition, they have other defects in that they tend to generate fog in a development-processing step, as well as upon storage, and undergo considerable deterioration in sensitivity and gradation during storage.
To prevent such phenomena, particularly the generation of fog, heterocyclic mercapto compounds called antifoggants, including 1-phenyl-5-mercaptotetrazole as a representative example, have so far been used very often. However, antifoggants of this kind adsorb so strongly to silver halide grains that they tend to adversely influence the photographic characteristics, e.g., they retard the progress of development, they lower the maximum image density, they decrease the contrast, and so on. Therefore, many restrictions exist in their use. Under these circumstances, it has been strongly desired to develop high chloride-content emulsions which have high sensitivity and, what is more, have reduced fog generation extent without using such antifoggants.